The trend of a liquid crystal display device such as a mobile phone, a personal digital assistant (PDA), a laptop and a tablet is going thinner and lighter, portable and multifunctional with the development of electronics industry. Energy consumption of a display of the liquid crystal display device is around 60%-70% of the entire consumption of the liquid crystal display device. In the display, energy consumption of a backlight module is 70%-80% of that of the display. However, a size and capacity of a battery in the liquid crystal display device is restricted by the volume of the liquid crystal display device. Therefore, energy provided to the backlight module is generally sacrificed for extending endurance of the liquid crystal display device when a battery is finite in size and capacity. However, brightness of the liquid crystal display will be reduced due to the sacrifice of energy consumption of the backlight module, so that poor effects will be provided in display.